Field effect transistors based on organic semiconductors are of interest for a multiplicity of electronic applications requiring extremely low production costs, flexible or unbreakable substrates, or the fabrication of transistors and integrated circuits over a large active area. For example, organic field effect transistors are suitable as pixel control elements in active matrix screens. Such screens are usually produced with field effect transistors based on amorphous or polycrystalline silicon layers. Typically, temperatures of more than 250° C. are necessary for fabricating high-quality transistors, which are based on amorphous or polycrystalline silicon layers requiring the use of rigid and fragile glass or quartz substrates. By virtue of relatively low temperatures (e.g., usually less than 200° C.) at which transistors based on organic semiconductors are fabricated, organic transistors permit the production of active matrix screens using inexpensive, flexible, transparent, unbreakable polymer films providing considerable advantages over glass or quartz substrates.
A further area of application for organic field effect transistors is the fabrication of very inexpensive integrated circuits that are used, for example, for the active labelling and identification of merchandise and goods. These so-called transponders are usually produced using integrated circuits based on monocrystalline silicon leading to considerable costs in the construction and connection technology. Producing transponders on the basis of organic transistors would lead to enormous cost reductions and could assist transponder technology en route to a worldwide breakthrough.
The fabrication of thin-film transistors usually requires a large number of steps in which the different layers of the transistor are deposited. In a first step, the gate electrode is deposited on a substrate, then the gate dielectric is deposited on the gate electrode, and the source and drain contacts are patterned in a further step. Finally, the semiconductor is deposited between the source and drain contacts on the gate dielectric.
Therefore, great effort is being made to simplify the fabrication process for field effect transistors. For example, available literature describes the use of printable molecular etching masks for pattern definition in the processing of organic transistors. For this purpose, an extremely thin molecular monolayer of a suitable organic material is applied to a metal layer and deposited over the whole area on the substrate, by a conformal relief stamp. In this case, the molecules are transferred to the metal in the regions of stamps in which the elevated structures of the stamp make contact with the metal surface. This form of relief printing is also referred to as microcontact printing or as flexographic printing. Such a method is described by J. L. Wilbur, A. Kumar, E. Kim and G. M. Whitesides (“Microfabrication by Microcontact Printing of Self-Assembled Monolayers”, Advanced Materials, 1994, 6, 600–604). The organic molecules used in this case are ideally configured such that chemical bonds and a molecular self-assembled monolayer (SAM) form between the individual molecules and the metal surface. The molecular structures defined on the metal surface in this way serve as an etching mask in the subsequent process step and thus permit the targeted patterning of the metal layer by means of wet-chemical etching methods. Once the molecular monolayer has fulfilled its task as an etching mask, it is removed again in order to uncover the metal surface for the next process steps.
When the metal layer patterned in this way is used as a gate electrode of the transistor, the process that directly follows the removal of the molecular etching mask is the deposition of the gate dielectric for the purpose of electrically insulating the gate electrode from the organic semiconductor layer, which is deposited in a further process. Inorganic oxides or nitrides, such as, silicon oxide, silicon nitride, aluminum oxide or tantalum oxide, or insulating polymers, such as, polyvinlyphenol, are generally used as the gate dielectric in organic transistors.
The processing of gate dielectrics based both on inorganic oxides and nitrides and on insulating polymers generally requires relatively large layer thicknesses of approximately 100 nm or thicker and therefore necessitates relatively high supply voltages for operating the transistors. The supply voltages are in the region of approximately 10 volts or higher. In principle, although the supply voltages can be reduced by using thinner gate dielectrics, this reduction of the layer thickness in the case of the conventional dielectric materials, mentioned above, inevitably leads to an unacceptable increase in the leakage currents and generally to a reduction of the yield.